Laser scanning systems have been developed that utilize a scanning laser beam to either write information to or read information from a scanned target. A typical example of a laser scanning system that reads information is a point-of-sale cash register system that scans bar code labels provided on products. Typical laser scanning systems which write information include laser printers that write on electrostatic photoreceptors or photographic materials.
U.S. Pat. No. 4,904,034 issued to Narayan et al., for example, discloses a laser scanning system that writes information to a target station. The laser scanning system includes a laser diode which generates the write beam, an electronic laser driver that modulates the write beam by varying the laser current and a holographic scanning disc that is rotated by a motor. The collimated write beam is directed onto the rotating holographic scanning disc which diffracts the write beam. An F-theta lens following the scanning disc focuses the diffracted write beam on the target.
It is desirable, however, to increase the throughput rate of printers employing laser scanning technology while maintaining printing resolution capabilities. One approach to increasing the throughput rate would be to provide multiple laser scanning beams that would scan several image lines simultaneously. For example, a multiple laser diode array could be provided to generate multiple beams that would be scanned in a conventional manner. The use of a multiple laser diode array, however, would require that each laser diode have equivalent operating characteristics to prevent line-to-line variations in the scanned image. Slight variations in wavelength, power, beam divergence angles or beam aspect ratio within respective laser diodes results in the appearance of "banding", i.e. visible lines, in the produced image. The requirement for equivalent operating characteristics also makes it difficult to produce an acceptable multiple laser array at a reasonable cost.
Another approach to providing both an increased throughput rate is to generate multiple independently modulated scanning beams by using multiple RF carriers in a single acousto-optic device. Acousto-optic devices, however, are limited in bandwidth. It is also relatively difficult to equalize beam power and minimize crosstalk in such devices between the multiple scanning beams.
Accordingly, it is an object of the present invention to provide a scanning device having increased throughput for a given level of printing resolution. It is a further object to provide a device for generating a plurality of scanning beams that can be independently modulated.